IN THE ANIMAL KINGDOM. 73 



cess of cell-formation occurs in the ova of Pyrosoma and 

 Salpa* 



The embryo whether the direct or indirect product of 

 impregnation differs in the majority of cases from the 

 adult form, not only in size, but in many points also of 

 structure and configuration, and the progressive changes 

 which it undergoes before acquiring perfect conformity to 

 the organization of the parent go under the name of meta- 

 morphosis. Sometimes they occur within the embryonic 

 envelopes, but at other times the young is extruded while 

 still in an imperfectly developed state. Such naked em- 

 bryos are termed larvce masks, as it were, disguising 

 what is ultimately found to be the true aspect of the species. 



In some Invertebrata, Fishes, and Reptiles, and still more 

 strikingly in Birds, the segmentation appears not to extend 

 to the whole contents of the ovum, but the exception is 

 more apparent than real, for the yolk of the fertilized egg 

 in such cases contains part of the granular matter within 

 the ovisac, over and above the proper substance of the 



* Annals of Nat. History, 3d Ser. (Jany., 1860), p. 35. There is per- 

 haps more force in Mr. Huxley's other suggestion of an analogy with the 

 Bird's ovum, though in this view the part wanting would not be the true 

 cleaving yolk, but the wall of the proper germinal vesicle, and the true 

 ovum would be represented by what he calls the germinal vesicle, not by 

 the unwalled contents of the ovisac, which would correspond rather to the 

 adventitious or yolk-food of the Bird. The homologies would stand as 

 follows : 



Vitelline membrane wanting. 



Food yolk liquifying yolk of Huxley. 



3 Temporary zona of Meckel \ ^J& " j?f^ 



'I 



True or primitive central ^ n 



__i* > Contents of above. 



granular yolk j 



Wall of germinal vesicle wanting. 



Macula of do germinal spot. 



Reasons will be given afterwards for regarding the wall of the germinal 

 vesicle as a non-essential structure, though one certainly of great con- 

 stancy in the animal kingdom. 



E 



